There are certain hydraulic systems which require high volume flow at low pressure as well as low volume flow at high pressure. Such a requirement occurs, for example, in a system in which a piston moves freely until it encounters a load or reaction member which imposes a relatively high resistance to further motion of the piston. In such an arrangement, it is desired to have a pump system which delivers high volume flow at low pressure to provide for free travel of the piston over a relatively large range and then delivers low volume flow at high pressure for piston displacement over a small range of travel. This would allow high speed motion during the free travel and the exertion of a relatively large force over a small range of travel and at a standstill.
Hydraulic brakes on automotive vehicles are commonly provided with a power booster to assist the driver in the application of the brakes. Hydraulic brake systems with a power booster, known as "power brakes", conventionally include a servo motor called a "booster" or augmenting the force applied by the driver to the piston of the master cylinder. It has been a common practice to utilize a vacuum powered booster on vehicles having spark ignited engines because of the availability of intake manifold vacuum for energizing the booster. However, on many present day vehicles it is desired to use a hydraulic booster with an electrically energized hydraulic pump. Such is the case with diesel engine vehicles which have no convenient vacuum source. It is also desired to have an electrically powered booster for other reasons such as having booster power with the engine off.
In a hydraulic brake system the flow requirement during the initial brake pedal travel is different from that during the final pedal travel. The system requires a high volume, low pressure flow during the free travel of the movable brake members and then when the brake members, e.g. brake pad and disc are engaged, the system requires high pressure, low volume flow to exert the braking effort.
In the prior art, it is known to use a motor driven, hydraulic booster pump with the motor being energized from the vehicle battery. In this arrangement, the pump is a rotary vane or piston pump and hydraulic fluid under pressure from the pump is stored in an accumulator. The pump is turned on and off in response to accumulator pressure in order to meet the flow requirements of the brake system. This is disadvantageous in that it requires a pressure sensing switch and an accumulator and furthermore the motor is turned on at times not necessarily coincident with the application of the brakes resulting in an on/off cycle which is distracting to the driver. It is also known in the prior art to maintain a continuous flow of fluid from the pump, which is energized from the vehicle engine or an electric motor, and to impose restriction on the flow to obtain the pressure required to actuate the booster.
A general object of this invention is to provide an improved hydraulic pump system capable of high volume, low pressure and low volume, high pressure operation, and which overcomes certain disadvantages of the prior art.